Nuclear reactor fuel and fuel element and preparation thereof

ABSTRACT

A NUCLEAR REACTOR FUEL AND FUEL ELEMENT THEREFROM COMPRISING URANIUM OXYCARBON CONTAINING 500-2500 PARTS PER MILLION OF OXYGEN HAVING THE EMPIRICAL FORMULAR   UC1.00O.01-.05   PREPARED BY HEATING SINGLE PHASE UC AT A TEMPERATURE IN THE RANGE OF ABOUT 2000* K-2400*K., PASSING HELIUM CARRIER GAS CONTAINING CO AND CO2 OVER SAID HEATED UC AT A SUFFICIENTLY HIGH RATIO OF CO TO CO2 TO PREVENT FORMATION OF UO2, SUCH AS AT 10,000:1, AND FOR A TIME TO PRODUCE SAID PRODUCT.

United States Patent O 3,714,060 NUCLEAR REACTOR FUEL AND FUEL ELEMENTAND PREPARATION THEREOF Marvin Tetenbaum, Hinsdale, and Paul D. Hunt,Lemont,

Ill., assignors to the United States of America as represented by theUnited States Atomic Energy Commissron No Drawing. Filed Feb. 12, 1971,Ser. No. 115,121 Int. Cl. C01g 43/00 US. Cl. 252301.1 R 5 ClaimsABSTRACT OF THE DISCLOSURE A nuclear reactor fuel and fuel elementtherefrom comprising uranium oxycarbide containing 500-2500 parts permillion of oxygen having the empirical formula prepared by heatingsingle phase UC at a temperature in the range of about 2000 K.2400 K.,passing helium carrier gas containing CO and CO over said heated UC at asufficiently high ratio of CO to C0,; to prevent formation of U0 such asat 10,00011, and for a time to produce said product.

BACKGROUND OF THE INVENTION The invention relates to a nuclear reactorfuel comprising uranium carbide containing oxygen combined therewith,also to the preparation of such fuel, and to fuel elements, particularlysteel clad fuel elements made with such oxygen containing uraniumcarbide fuel.

Uranium carbide is well known as a fuel for nuclear reactors, especiallyof the breeder types. Such fuels are generally in the form of blocks orcylinders clad with steel, usually a stainless steel. However, uraniumcarbide fuels which have hitherto been made have either contained highcontents of oxygen, such as .5% or higher, in the form of U0 or havebeen substantially pure single phase UC containing less than 380 partsper million of oxygen, as described in US. Pat. 3,392,005.

One big disadvantage of uranium carbide fuel is the transfer of carbonto the cladding, particularly to the steel cladding of clad fuelelements containing uranium carbide. Any uranium carbide fuel whichwould have increased stability and decreased tendency of carbon transferover those uranium carbide fuels above mentioned and hitherto known tothe art would be very desirable.

SUMMARY OF THE INVENTION Applicants discovered that there was a criticalcontent of oxygen combined with uranium carbide which gave decreasedcarbon activity and hence a decreased tendency of the carbon of theuranium carbide to react with the cladding material, particularly steelcladding of the clad fuel element. Such an oxygen content was found tobe 500 2500 parts per million of oxygen, corresponding to a uraniumoxycarbide having the empirical formula This fuel contains no U and noU0 The uranium oxycarbide fuel of this invention is prepared bycontacting single phase UC (no U and no U0 with a mixture of CO and COpreferably in an inert carrier gas such as helium or argon at a ratio ofCO to CO sufficiently high to prevent formation of U0 such as a range ofl0,000-100,000 to 1, at a temperature in the approximate range of 2000K.-2400 K. for a time sufiicient to complete the formation of UC O suchas for 3-10 hours or longer, the reaction being carried out with acarrier gas having subatomspheric partial pressure of C0 of, forexample, 4-10 torr (balance He). Temperatures below 2000 K. may be usedbut this increases the time, and while temperatures above 2400 K. can beused the results are nonuniform and such high temperatures are notdesirable for best results. It is preferred that helium carrier gascontaining the CO and CO be continuously passed over the heated UC untilthe reaction is completed.

The key to obtaining the low oxygen addition to the UC is the use of amixture of CO and CO having a very high ratio of CO to CO such as 30,000to 1. Applicants have been able to readily adjust the CO feed by asample analysis technique whereby CO is cold trapped (frozen out of thegas mixture) at 78 C. on silica gel with subsequent quantitativeanalysis by gas chromatography.

The uranium oxycarbide fuel produced by applicants method not only hasthe important critical limitation of oxygen of 500-2500 parts permillion, which gives decreased transfer of carbon to cladding, but alsohas the advantage of being free from U and U0 both of which areundesirable in UC fuel elements.

The carbon activity of UC O produced by our process is approximately0.001 as compared to 0.15 for single phase UC This carbon activity isregarded by ap plicants as a measure of the tendency of the carbon ofthe UC to transfer to cladding material such as steel. The carbonactivity of the uranium carbide produced by the process of US. Pat.3,392,005 is close to that of UC and several orders of magnitude greaterthan uranium oxycarbide of this invention. With uranium oxycarbidecontaining U0 the carbon activity is substanitally greater than forapplicants uranium oxycarbide, and applicants absence of U0 is anadvantage for nuclear reaction fuel which should contain as high UC aspossible commensurate with low carbon activity.

DESCRIPTION OF SPECIFIC EMBODIMENTS In our experimental work in carryingout the process of this invention considerable effort was made to purifythe carrier gases 'CO, argon and helium to minimize contamination fromCO H 0 and oxygen. To avoid the formation of U0 when investigating theUCO region, the oxygen partial pressure must be lower than that neededfor the formation of U0 Therefore very high CO/CO ratios are required.With a gettering system consisting of a combination of hot copper and amolecular sieve, we were able to obtain a CO He carrier gas mixturecontaining less than 0.1 part per million CO Oxygen partial pressure,carbon activity, and total pressure of uranium-bearing species weremeasured at 2255 and 2355 K. with a carrier gas having a CO partialpressure of -10 torr (balance helium). The uranium charge used in ourstudies consisted of a spherical 8+12 mesh powder with the followingcomposition: -4.55 wt. percent carbon (C/U=0.96), p.p.m. oxygen, -350p.p.m. nitrogen, and 200 p.p.m. metallic impurities. The results aresummarized in Table I. Recent experiments at high cartier-gas flowratesminimized the discrepancy between input and output CO/CO ratios.Metallographic examination of the residues did not show the presence ofUO (s) in the matrix or on the surface of the particles in any of theexperiments.

The lower oxygen content (about 1 atomic percent) of the oxycarbideresidue, when compared with previous results (about 2 atomic percent:Run 100-1 given in table for comparison and also shown in Table 1), ap-

5 pears to be consistent with the lower oxygen partial pressure (higherCO/CO ratio) of the carrier gas used TABLE L-VALUES OF CARBON ACTIVITYAND OXYGEN in the later runs. The slightly lower values for the cal-PARTIAL PRESSURE FOR WG- SYSTEM culated carbon activity reflect thelower CO partial pres- RIIH 100-1 sures used in the present runs.Consistent with previous Temperature K 2,255 2,255 2,355 10 results, theeffect of a small addition of oxygen is a 52 3 1 hour- CO i Co 00 i 5significant reduction in the activity of carbon. Flow ,;.5, 1 8 8 1 8 Arough estimate of the free energy of formation of Partialpressure CO inthe oxycarbide composition UC O at 2355 K. was

carrier gas, torr -10 -10 -10 CO/COz, input gas 13x10: 113x105 333x104made from values for oxygen partial pressure, carbon ESQ- 3 gggggg i 111 5 1 1 3 :3 0 15 activity, and total pressure of uranium-bearingspecies on Output) 1218 1228 1220 derived from our measurements. In theabsence of spegagfif i i fi 0.011 0.011 2' 0X 1H cies information, theuranium activity was roughly esti- Carbon activity (based on mated basedon the total pressure of uranium-bearing cgr ii iiihiliiiIIlIIIIIIiIIIco i ua mfiggffi gj figj species, the assumption that the vapor isuranium gas, Nitrogen content, .m 99 7 and the subhmatron pressure ofpure uranium. From Metamgmphic examination" W (1) the expression AG =R'Tlog (Ngta a tentative value 1 U02 not detected. of AG =-41.6 kca1./molwas obtained The embodiments of the invention in which an exclusiveproperty or privilege is claimed are defined as follows:

From the data in the above Table I we calculated 1. The process ofmaking a nuclear reactor fuel comcarbon activities to be between 10" and10- at 2255 prising contacting UC with a mixture of CO and CO K., andapproximately 2 10 at 2355 K. For comparihaving a ratio of CO to CObetween about 10,000:l and son, the carbon-activity values, based on ourmeasure- 100,000:1 at a temperature in the approximate range ments withH CH mixtures over oxygen-free UC of 2000 K.-2400 K. for a timesufficient to complete compositions ranging from C/U=0.99 to C/U=1.00,the formation of a uranium oxycarbide having the emyielded values of aranging from 0.09 to 0.15, respecpirical formula UC O tively, at 2255K., and -0.15 at 2355 K. The effect 2. The process of claim 1 whereinthe mixture of C0 of a small addition of oxygen, e.g., 1.5-2.0 at.percent, with CO contains an inert gas. results in a significantreduction in the activity of carbon. 3. The process of claim 1 whereinthe mixture of gas In an additional series of experiments oxygen partialincludes the carrier gases CO and He and the mixture pressure, carbonactivity, and total pressure of uraniumis passed continuously over theUC. bearing species were measured at 2355 K. with a carrier 4. Theprocess of claim 3 wherein the C0 of the gas having a CO partialpressure of -4 torr (balance carrier gas has a partial pressure of 4-10torr, the balhelium) and a CO/CO ratio of -5 10 The composition of theuranium carbide charge was C/U=0.96. The results are summarized in TableII.

ance being He.

5. The process of claim 1 wherein the time is 3-10 hours.

U-C-O SYSTEM Run 100-1 101-1 101-2 Temperature, K 2, 355 2,355 2, 355Time of run, h0urs.... 5.5 2. 9 6. 7 Flow rate, literslminun 1. l0 1.08 1. 08 Carrier gas CO-HB CO-COz-He CO-COz-He Partial pressure CO incan r gas, tor -10 4 4 (JO/CO2, input 312x10 5.4)(10 5.7)(10 COICOz,output 2. 2x10 4. 6x10 4. 7x10 log p(Oz) (based on input), atm 12. 5212. 98 12. 94 -log p(Oz) (based on output), atm. 12. 20 12. 12. 78Carbon activity (based on input). 2. 0X10 1. 3 10- 1. 4X10- Carbonactivity (based on output) 1. 4X10 1. 0X10- 1. 1X10 log p(U) totalpressure of uraniumbearing species, atrn 5. 61 5. 83 COmIJOSi iOHUCmoOnm UCmoOo oz UCmnOMz Metallographic examination 1 U02 not detected.

References Cited UNITED STATES PATENTS 3,386,811 6/1968 Hanson 23-349 X3,398,098 8/1968 Hanson 25230l.1 3,219,541 9/1965 Bradley 176--67FOREIGN PATENTS 1,052,084 12/1966 Great Britain 23349 OTHER REFERENCESRussell et a1, Carbides in Nuclear Energy, vol. 2, 1964, Macmillian &Co., Ltd., London, pp. 674, 675, 73840.

CARL D. QUARFORTH, Primary Examiner R. L. TATE, Assistant Examiner US.Cl. X.R.

